1. Field of the Invention
The present invention relates to an improved process for the separation of xylene isomers. Further, this invention relates to an improved method of separating para-xylene from C.sub.8 aromatic mixtures by the use of an immiscible solvent.
2. Discussion of the Prior Art
Para-xylene because of its utilization in the production of terephthalic acid and dimethylterephthalate, both of which are converted into polyester fiber and film, is the C.sub.8 aromatic isomer currently in greatest demand. The other xylene isomers are also of some economic importance but are in less demand as chemical intermediates. Thus, while ortho-xylene, meta-xylene, and ethylbenzene are used in the production of phthalic anhydride, isophthalic acid, and styrene, respectively, these isomers are more typically isomerized to produce additional amounts of the more desired para-xylene.
A typical C.sub.8 aromatic feedstock which contains all of the C.sub.8 aromatic isomers in varying quantities is not readily separated by fractional distillation into all of the individual isomers. Ortho-xylene, which has a boiling point 3.5.degree. C. higher than that of its closest boiling C.sub.8 aromatic isomer (meta-xylene), can be separated by conventional fractional distillation techniques. Such ortho-xylene towers will contain 100 to 150 trays and will operate with about a 5-8 to 1 reflux ratio. Ethylbenzene can be separated with increased difficulty from such a C.sub.8 aromatic mixture but only by more intricate superfractionation since the boiling point is within 2.2.degree. C. of the boiling point of para-xylene. Typical ethylbenzene fractionators contain 300 to 400 actual trays and require about a 25-50 to 1 reflux to feed ratio. Since the meta and para-xylene differ by only 0.7.degree. C. in boiling point, separation of these isomers by distillation is essentially impossible and other means have to be employed.
Although selected sulfonation and HF-BF.sub.3 processing schemes have been employed for separating para and meta-xylene, the most commonly encountered commercial technique for separating meta and para-xylene is fractional crystallization, a separation method well known to the art. Unfortunately, complete recovery of high purity para-xylene from a given feed stream is impossible by fractional crystallization because of the eutectic formed between meta-xylene and para-xylene. In fact, 98+ percent purity para-xylene can be recovered from typically encountered refinery streams in only about 60 percent para-xylene recovery per pass through the crystallization zone.
It is to such a separation as this, that selective adsorption techniques are well suited and, indeed, adsorption techniques utilizing crystalline aluminosilicates have been used successfully to separate meta and para-xylene. U.S. Pat. Nos. 3,133,126 and 3,114,782 to Fleck; U.S. Pat. Nos. 3,558,730, 3,558,732, 3,626,020, 3,663,638 to Neuzil and U.S. Pat. No. 3,665,046 to DeRosset and U.S. Pat. No. 3,668,266 to Chen. et al. are illustrative examples. Selective adsorption has the advantage of being able to produce high purity para-xylene in higher yields (about 85-90 percent or higher) than those obtainable by fractional crystallization.
In a particular selective adsorption process, such as that described in U.S. Pat. Nos. 3,558,730, 3,558,732, 3,626,620, 3,663,638 and 3,665,046 a feed mixture containing C.sub.8 aromatic isomers contacts a bed of crystalline alumino-silicate adsorbent to effect the selective adsorption of a first C.sub.8 aromatic component. A raffinate stream comprising less selectively retained xylene-isomers is withdrawn from the adsorbent bed. The adsorbent bed then is contacted with a desorbent material to remove the selectively adsorbed first C.sub.8 aromatic component from the adsorbent, and the desorbed C.sub.8 aromatic in admixture with desorbent is withdrawn from the adsorbent mass. The desorbent is then fractionated from the raffinate and extract streams for subsequent reuse in the process. The selectively adsorbed isomer is usually para-xylene.
The most commercially used process, "UOP's Parex Process," is based on a continuous selective adsorption in the liquid phase, employing a fixed bed of solid adsorbent. This solid adsorbent, made from zeolite material that has undergone exchange with barium and potassium, permits entry into the pore structure of the main feed components. The desorbent employed is neither weakly nor strongly adsorbed with respect to the C.sub.8 aromatic feed components. Preferred desorbents are toluene or paradiethylbenzene either of which can be readily recovered by distillation from the para-xylene. The open structure of the solid adsorbent, exposing a relatively large surface area to the feed, gives access to more adsorptive sites than if the adsorption were limited to the exterior surface. Para-xylene is the most readily absorbed material. The desorbing liquid can be readily separated from the feed components by distillation. The desorbent is adsorbed by the adsorbent to about the same extent as the feed hydrocarbons. It must be capable of being desorbed by feed hydrocarbons which it displaces by mass action. The continuous process operates with a fixed bed, which appears to move in the opposite direction from the liquid streams.
It is therefore one object of the present invention to provide an improved method of separation of liquid mixtures.
It is another object of this invention to provide an improved process for the separation of para-xylene from C.sub.8 mixtures.
A further object of the invention is to improve the economics of a known separation process by the use of a solvent immiscible with the products and the feed so to eliminate the need of distillation.
A still further object of this invention is to use a solvent in order to alleviate air and waste pollution.
The achievement of these and other objects will be apparent from the following description of the subject invention.